The present invention relates to a tape recorder for helically recording, reproducing or erasing signals on a tape in a magnetic or optical manner.
In order to record signals on a tape helically, there have generally been used tape recorders, such as home video tape recorders (VTRs) using magnetic recording media, in which a magnetic head built into a drum is rotated at a high speed and a tape is helically moved over the drum at a lower speed. Meanwhile, in tape recorders for recording, reproducing or erasing signals on optical recording media by the use of a laser beam condensed to fine spot, if an optical head is built into a drum, the centrifugal force produced upon rotation of the optical head at a high speed would adversely affect an actuator of the optical head, and the recorder structure would be increased in size. For such reasons, as disclosed in Japanese Patent Publication No. 63-869, there has been proposed a tape recorder in which tape feed and take-up spools are arranged one above the other and a tape is helically moved round a drum with an optical head fixed outside the drum for recording, reproducing or erasing signals.
In any of the conventional tape recorders as mentioned above, while the tape is helically wound round the drum, it is not wound throughout tee outer periphery of the drum, leaving some region where the tape is not present.
When recording or reproducing signals on a tape with high density, a head is required to accurately follow a signal track. In video tape recorders using magnetic recording media, as shown in FIG. 11, because a magnetic head has a wide width and a tape is held substantially in contact with the magnetic head for recording, reproducing or erasing, the recorder has been operated stably regardless of the partial absence of the tape from a drum. In an attempt of achieving higher-density recording in future use, however, it is required to accurately track a target signal track (called tracking). In this case, if the target track is partially disappeared, signals could not be recorded, reproduced or erased in a stable manner.
Meanwhile, in the case of tape recorders using optical recording media, if the tape is partially absent from the drum, focusing servo and tracking servo of the optical head becomes unstable, and this also makes it impossible to record, reproduce or erase signals in a stable manner.
Further, where the tape feed and take-up spools are arranged one above the other in the drum, the level of the tape edge differs at least a tape width between inlet and outlet points of the tape with respect to the drum. Therefore, the signal track to be obtained by the head disposed inside or outside the drum and scanning the tape helically wound round the drum cannot be formed throughout the outer periphery of the drum. Accordingly, when tracking the target signal track, the signal track is necessarily disappeared over a larger region than that where the tape is not present, also making it impossible to record, reproduce or erase signals in a stable manner.
In addition, the inventors have previously proposed an optical tape recorder as disclosed in U.S. Pat. Ser. No. 239,389 (filing date; Sept. 1, 1988,now U.S. Pat. No. 4,970,707). The proposed optical tape recorder has a guide plate disposed between a rotating drum and a tape for keeping stability of tape running, to thereby restrain fluctuations while the tape is running. Also, automatic focusing with high rigidity endurable to large centrifugal force caused by high-speed rotation is carried out using automatic focusing by utilizing the technology to vary the focus position of a laser beam with changes in the wavelength of a diode laser, or automatic focusing due to an actuator made of a piezoelectric element. Further, an additional head is provided separately from an optical head for recording or reproducing information, so that positional information to detect a running speed of the tape is recorded on the tape with certain intervals. That positional information is employed to record track guide information at the target locations precisely. Based on signals of the track guide information, the optical head for recording or reproducing the information can record signals and addresses of the information at the target locations precisely.
In the above prior art, however, no considerations are paid to the fact that the tape is not present over the drum in some region, and hence the head cannot be stably positioned on the signal track. This caused a problem in a control system for positioning the head.